(1) Field of the Invention
This invention relates to snap acting thermally responsive bimetallic actuators for use in actuating the switch contacts of thermally responsive electric switches such as cut-outs, circuit breakers and thermostats.
(2) Description of the Prior Art
A well known form of bimetallic actuator is a bimetallic disc of domed configuration which moves to an oppositely domed configuration with a snap action with changes in temperature. The simplicity of discs and their ease of manufacture, the basic operation being pressing a flat disc between steel dies to a desired domed configuration, has resulted in their widespread use, despite serious disadvantages.
One disadvantage particularly is that with prior art wide differential discs very high peripheral stress concentrations result during inversion and in consequence the operating temperature at which the disc snaps tends to drift with age and its life is limited by stress-cracking: thus after about 10,000 operations the disc may stress-crack, whilst the operating temperature may have drifted by as much as 20.degree. C. The net result is that in use such discs do not have accurately determined operating temperatures throughout their life.
Another disadvantage of prior art disc type bimetalic actuators is that of their very small range of useful movement with snap-action which is often of the order of the uncertainties in the dimensions of other related components of a switch in which the disc may be incorporated.
In view of the widespread use of discs, extensive research has been made into their characteristics. There have been various proposals over the past 40 or so years to increase the movement and to increase the accuracy of the operating temperature of discs. For example, radial corrugations have been impressed in a disc, stress-relief apertures have been inserted at the centre of a disc and mechanical arrangements for transmitting and increasing the effective movement of a disc have been proposed (see for example British Patent Specification No. 1031827). Despite the large amount of research that has gone into improving the characteristics of discs, none of the various proposals has done anything more than mitigate to a certain extent the disadvantages inherent in bimetal discs.
Another type of snap-acting prior art bimetallic actuator, which is well known and which avoids most of the disadvantages of discs, is that described in British Patent Specification No. 657434. In the preferred form, such an actuator comprises a rectangular sheet or blade of bimetal having a central tongue released from between two outer legs whose ends adjacent the free end of the tongue are joined by a bridge portion. The bridge portion is mechanically crimped to impart a dished configuration to the bimetal blade so that it moves with changes in temperature between oppositely dished configurations with a snap action. Such an actuator has a much larger amount of movement with snap action than a disc. It is also possible accurately to set the operating temperature, which is in any case more stable under repeated use than that of a disc.
The main disadvantage of the actuator of British Patent Specification No. 657434 inherent in the method of deforming the blade by crimping, is that it cannot be mass produced to a closely predetermined operating temperature, and furthermore the life of this actuator, although greater than that of a disc, tends to be shortened by cracks developing in maximum stress concentration areas at the junction of the central tongue with the outer legs.
Thus despite extensive research into and extensive use of various snap-acting bimetallic actuators the need still exists for a thermally responsive snap-acting bimetallic actuator which is suitable for mass production, has a reasonably stable operating temperature over a long working life, and provides an adequate range of useful movement with snap action.